Previously, many clamping devices have been developed in which a workpiece or the like is releasably secured between a fixed member and a movable member powered by a hydraulic or pneumatic cylinder. When the workpiece is clamped in the device, the cylinder continuously yieldably urges the members into engagement with the workpiece. This permits additional displacement of the movable member when needed without allowing the workpiece to become loose or be subjected to a substantially decreased clamping force. This need for additional displacement after initial clamping of the workpiece can be caused by many things, including expansion and contraction of the workpiece and members of the clamping device due to changes in temperature. The continuous bias provided by the cylinder also compensates for tolerances and variations in dimensions of a series of successive workpieces secured by the clamping device.
In use, hydraulic and pneumatic clamping devices utilize relatively large amounts of energy, must be supplied with fluid under pressure continuously even though they may not be used at any given time, produce considerable heat, and usually contaminate the environment in which they operate with oil which collects much dirt. In operation, these devices and particularly pneumatic devices, are objectionably noisy.
In recent years, laws and regulations have been promulgated establishing standards for maximum noise and contamination levels. It is most difficult, and in many instances impractical, if not imposible, to construct hydraulic and pneumatic clamping devices which comply therewith.